Scope of Works and Best Practice for Concrete slab home extension

https://www.apollohomes.net.au/bestpracticeguidefloorintersections/

Adding more floor area to your home involves more than just pouring a new concrete slab up against your existing property. Whether your existing home is built on a re-enforced concrete slab or a timber suspended floor, we always recommend using a re-enforced concrete slab as the foundation for your extension (Unless you are on a sloping site or specific circumstance where a Stump, bearer and joist flooring system required). Not only is concrete a structurally stronger foundation, but if constructed in compliance with AS 2860 (Residential Slabs and Footings) or AS 3600 (Concrete Structures), it is also an effective Termite management system when a chemical termite barrier is applied under the slab prior to placement of Waterproof membrane and concrete and re-enforcement.

Any control joints, penetrations through the slab and certain footing slab joints would still need to be provided with a termite management system. This could be either a chemical, granular or sheet material. We recommend using HomeGuard TMBinstalled by an accredited installer.  

Below is the scope of works required for floor junctions/Intersections from existing floor structure to new floor structure (Concrete floor has been used for this example’s existing floor structure). Although the steps involved incorporate Scope of works required for preparing and pouring the extensions’ new concrete slab, the approximate cost includes only what is required to Join existing slab to the new slab.

Intersections – Floor – Join Slab
See figure 1.2

+ Remove existing exterior skirt material (Paving or concrete hardstand)
+ Remove soil and replace with clean fill, bring up to required Subbase level (if required by engineer to achieve compaction, and based on soil classification of site)

Best practice note: To ensure minimum slab thickness is achieved, add
an extra 10% depth to engineers slab thickness requirements (100mm
min is our recommendation) to allow for subbase deviations.

+ Compact the new extension area, plus an additional 1200mm beyond extension area (to be tested prior to concrete preparation to ensure adequate compaction as required by engineer and council requirements)e.g. All ground beneath the building and extending 1200mm beyond to be          compacted to achieve a minimum of 7 blows per 300mm when tested with a Perth Standard Penetrometer (Unless noted Otherwise). Compaction tests to be carried out to the full depth of the fill material or to 750mm (whichever is greater)
+ Excavate footings as per engineers’ requirements (this includes thickening existing footing of building if required)

Best practice note: Compact footing with a ramming compactor and
ensure footings achieve same compaction min. as required for slab
sub base. Your builder should have a Perth Standard Penetrometer
 (PSP) to self-test.

+ Chemically anchor N12 x 450mm Long Deformed bar x 150mm into existing slab allowing 300mm to extend into new slab.
+AS 3600.1 - 2014 Compliant Termite Control/Cold joint treatment to be attached to slab cold joint intersection by licenced and accredited pest control expert.
+ AS 3600.1 - 2014 Compliant Termite Under slab sprayed termite pre-treatment applied (Note: If slab isn’t constructed in accordance with AS 2860 then a reticulated termite system must be provided under slab)
+ High Impact Resistant Polyethylene Film 0.2mm thick Damp proof membrane laid over compacted sub base in strict accordance with AS 2870, ensuring the bottom surface of the concrete slab and footing edges are entirely under laid by the film. The joints are to be overlapped by not less than 200mm to ensure continuity, and to be continuously taped using 50mm duct tape min. Care should be taken to ensure any penetrations by pipes or services are sealed to ensure continuous vapour barrier. All penetrations should also be sealed from termite penetration in accordance with AS 3660.1 – 2014.+ Expansion/Control Joint insert (100mm x 10mm Abelflex) attached to termite control joint, attached to existing slab prior to concrete pour, tear able 10mm strip removed after pour, and sealed with polyurethane sealant.
+ Reinforcement mesh sized per engineers specifications to be placed on bar chairs spaced at not more than 800mm centres. Special care is required to ensure adequate concrete cover between the outermost edge of the reinforcement and the surface of the concrete of not less than:        
- 30mm to Damp Proof Membrane
- 20mm to an internal surface (top of internal slab)
- 40mm to external exposure
+ Concrete at a minimum of 25MPa strength at 28 Days placed, 80mm slump unless specified differently by engineer for pumping (all concrete slabs should be pumped unless chute can directly access pour area, as wheelbarrowing of concrete increases risk of reinforcement mesh not achieving bottom cover due to weight of barrow squashing mesh prior to concrete placement).

Note: Although slabs up to 100mm aren’t required to be compacted using an immersion vibrator, we highly recommend the use of a vibrator throughout any pour to correctly consolidate the fresh concrete. Due to 80mm slump being a dryer mix, the use of an immersion vibrator has the added advantage of making the concrete easier to work with once vibrated.

+ Once concrete has been finished, the freshly poured and finished slab should be cured for at least 7-Days to ensure it has the opportunity to reach its maximum strength. Although concrete strength is measured at 30 days, the most important time for curing is the first 7 days. That is why it is also very important that edge formwork is kept in place while the slab is curing.

Best Practice note: When curing the slab with a plastic membrane, white or lightly coloured plastic sheets reflect the rays of the sun and, hence, help to keep concrete relatively cool during hot weather. Black plastic, on the other hand, absorbs heat to a marked extent and may cause unacceptably high concrete temperatures. Its use should be avoided in hot weather, although in cold weather its use may be beneficial in accelerating the rate at which the concrete gains strength. (CCAA DS2006Curing)

Approx Cost – Floor to Floor Junction Only: $196 L/M PLUS GST

Figure 1.2